Explain that scientists consider freshwater habitats to have salinity, or saltwater content, of less than .005 percent. Freshwater habitats are ponds, lakes, bogs, rivers, streams, creeks, marshes, swamps, or even puddles and drainage ditches. A reservoir is an artificial freshwater resource.

Brainstorm with students a list of freshwater habitats close to school. Ask students to hypothesize about the health of a nearby habitat and its diversity of life.Would students consider it healthy or unhealthy?Why? Explain that some organisms can tolerate a polluted freshwater environment.

Introduce the word "macroinvertebrate" to the class. Explain that this is an animal without a backbone living in one stage of its life cycle, usually the nymph or larval stage. Visible without a microscope, macroinvertebrates can spend a few years living in a freshwater habitat. Many are benthic organisms, or bottom dwellers.

Explain that scientists determine a freshwater habitat's health by the number and type of its organisms. The water quality of such a habitat is good when it is rich in oxygen and supports a variety of organisms. Water quality is fair when it has less oxygen and low levels of pollutants, and poor when it has high levels of pollutants. Some organisms can survive only in freshwater of good quality, while others can live in any quality.

Review the chart of organisms and the quality of water their presence indicates (below). The presence of stonefly larvae indicates good water quality because they are highly sensitive to chemical and physical changes. But leeches can live in any water, so their presence may indicate poor quality.

Good water quality

Fair water quality

Poor water quality

Mayfly larvae

Crayfish

Aquatic worms

Stonefly larvae

Scud

Leech

Caddisfly larvae

Dragonfly nymph

Pouch snail

Dobsonfly larvae (Hellgrammite)

Cranefly larvae

Midge fly larvae

Water penny

Clam

Blackfly larvae

Riffle beetle

Damselfly larvae

Carp

Trout

Sow bug

Catfish

Divide students into teams of three or four. Explain that students will study shallow and deep areas of the freshwater habitat. Shallow water in a stream is no higher than the ankle, and deep water reaches the knee.

Review safety precautions to follow during fieldwork. Wear old boots that will keep feet dry; remember wet surfaces, such as rocks with algae, are slippery; check depth before stepping into water; handle organisms gently and return them to the habitat alive; be aware that some organisms can bite or pinch; never drink the water.

Have each team gather equipment and choose a place to work in the habitat. Have students observe the water's edge and surface, and look through the water to the bottom of the habitat. Have the students record their observations on a data sheet.

Have students measure the water temperature with a thermometer and record depth with a meter stick. They will measure the velocity of flowing water by recording the distance a float travels in 10 seconds, and measure the water's pH with pH paper. They should record all data on their data sheets.

In a stream, have students collect macroinvertebrates in riffles, or areas with higher oxygen content.

Students should place a kick seine net or a large net with a small mesh downstream. Have them hold the net so it rests on the bottom of the stream.

Have students dislodge organisms by disturbing the stream bottom and rubbing rock surfaces. After a few minutes, students will raise their nets and gently put organisms into a bucket. In still water, students must carefully sift through mud or sand in the net. Students should identify and count the organisms, and record their data before releasing them.

EvaluationUse the following three-point rubric to evaluate students' work during this lesson.

Three points:Students worked exceptionally well in the field and completed data sheets accurately with detailed observations; answered the questions completely and shared observations with the class; demonstrated a clear understanding of the fieldwork.

Two points:Students worked somewhat carefully in the field and completed data sheets, but answers lacked detailed observations; completed most of the questions and demonstrated a general understanding of the fieldwork.

One point:Students did not engage in fieldwork and partially completed the data sheets; answered some of the questions, but did not demonstrate an understanding of the fieldwork.

StandardsThe National Academy of Sciencesprovides guidelines for teaching science as well as a coherent vision of what it means to be scientifically literate for students in grades K-12. To view the standards, visithttp://books.nap.edu.